1. Field of the Invention
This invention relates to a preheater of the type mounted within a well, and more particularly, to a heater adapted to preheat sodium or the like for use as a cooling medium in a nuclear reactor, by heating a pipeline defining a sodium passage.
2. Description of the Prior Art
The sodium which is circulated through a pipeline for cooling a nuclear reactor need be preheated to a desired temperature. It has hitherto been usual to provide a sheathed heater along the surface of the pipeline for sodium so that it may heat the pipeline to thereby preheat the sodium. The heater has its own life, and also requires maintenance and inspection from time to time, leading to the necessity for its dismantling and replacement. The heater is, however, located in the vicinity of a nuclear reactor, and it is very dangerous for a man to have any direct access to the location of the heater for doing the necessary work therefor. It has, therefore, been strongly desired to make it possible for a man to dismantle and replace any such heater without approaching such a dangerous place.
This desire can most effectively be satisfied by providing a fixed well tube along a pipeline for sodium, and securing a removable heater within the well tube. The worker has only to place an elongated heater into the well tube, or take it out, through a tube opening located at a sufficiently distant place from the reactor. He is not exposed to any danger of radioactive contamination when dismantling or replacing the heater. Although the use of a well tube for removable installation of the heater is very advantageous as hereinabove mentioned, the sodium pipeline connected to the reactor, which is never straight, but complexly bent, necessarily requires a well tube which is bent along the sodium pipeline. More specifically, the use of such a well tube for mounting a preheater for the sodium pipeline for a nuclear reactor has heretofore required the heater to satisfy the following conditions:
(a) As the sodium pipeline follows a route which is not only straight, but also complexly bent, the well tube installed along the sodium pipeline also follows a complexly bent route, and has a substantial total length. Therefore, the heater must be sufficiently flexible to be properly placed into, and out of the well tube.
(b) The heater must be able to withstand a temperature of at least 500.degree. C., because the temperature of the heater in the well tube is expected to exceed 400.degree. C. in order to preheat the sodium pipeline to a temperature usually in the approximate range of 200.degree. C. to 300.degree. C.
(c) The heater is required to permit not only easy insertion into the well tube, but also easy withdrawal therefrom.
(d) Since the sodium pipeline has such a temperature gradient along its length that in the vicinity of the reactor, its temperature is raised by the transfer of heat from the reactor, while that portion of the pipeline which is located outside the vessel which contains the reactor remains at room temperature unless preheated, it is necessary for the heater to have an adjustable output distribution in order to preheat the pipeline uniformly along its entire length.
(e) The heater is required to have a sufficiently high mechanical strength to withstand the frictional force, bending stress and torsion to which the heater is subjected when it is inserted into the well tube, or withdrawn therefrom.
No heater known in the art has satisfied all of the aforementioned requirements, but heretofore, the use of any such well tube has been employed for only a substantially limited portion of the sodium pipeline which extends in a straight line through the wall of the reactor vessel, despite the excellent advantages provided by the aforementioned method for mounting the heater. It is true that the area in the exterior of the wall of the reactor vessel contains less radioactivity, but it is never safe for anybody to engage for a long period of time in the work which has to be done in such an environment. It has, thus, been desired to find a solution to that problem.